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Atrial Septal Defect, Ostium Primum
Updated: Nov 7, 2008
Introduction
Background
An ostium primum atrial septal defect (ASD) is located in the most anterior and inferior aspect of the atrial septum. It is the simplest form of atrioventricular (AV) canal or AV septal defect. These defects are often associated with trisomy 21.
During fetal development, the rudimentary atrium is divided by the septum primum, except for an anterior and inferior space that is the ostium primum. The ostium primum is sealed by fusion of the superior and inferior endocardial cushions around 5 weeks' gestation. Failure to do so results in an ostium primum ASD. The endocardial cushions also contribute to the complete formation of 2 separate AV valves and the inlet interventricular septum. For this reason, ostium primum ASDs commonly are associated with malformations of these structures.
Ostium primum ASDs may occur in isolation but most commonly present with a cleft in the anterior leaflet of the mitral valve. This is sometimes termed a partial AV canal defect or a partial AV septal defect. In this case, a 5-leaflet AV valve is arranged so that separate right and left components (a tricuspid valve and a mitral valve) are present. The leaflets connect to each other and then adhere to the crest of the interventricular septum. This results in obligatory shunting at the atrial level with no ventricular level shunting. Generally, a commissure is observed between the left superior and inferior bridging leaflets because of abnormal fusion of the left tubercle of the superior and inferior cushions, which results in a cleft in the anterior leaflet of the mitral valve.
Pathophysiology
Shunting is predominantly left-to-right in the absence of pulmonary vascular disease or significant right ventricular outflow tract obstruction. This results in volume overload of the right atrium and ventricle and pulmonary overcirculation. If the mitral valve cleft causes significant mitral regurgitation, the left side of the heart also becomes volume overloaded. A left ventricle to right atrium shunt can be present, which further overloads both the right and left heart.
Frequency
United States
Ostium primum ASDs are most commonly associated with Down syndrome (trisomy 21). The incidence of trisomy 21 is 1 per 800 live births, with an increased prevalence noted in children born to older mothers.
- The overall risk of congenital heart disease in patients with Down syndrome is 40-50%. Approximately 65% of those affected have some form of AV septal defect.
- The inherited risk for children of parents who have an AV septal defect is reported as 9-14%.
Mortality/Morbidity
The presence and degree of associated mitral regurgitation and/or left ventricle to right atrium shunting generally determine symptoms.
Those with either no cleft or a cleft with a mild degree of mitral regurgitation are often asymptomatic. Patients typically are referred for evaluation of a heart murmur in childhood and generally survive well into adulthood. However, adults who have not had the condition repaired often become symptomatic from congestive heart failure (CHF) by age 45 years. Rarely, patients are reported to present in the seventh decade of life. Dyspnea on exertion and fatigue are usual adult complaints. Palpitations secondary to atrial fibrillation or flutter also are common.
Those with more severe mitral regurgitation or left ventricle to right atrium shunting often present in the first 2 years of life. Mortality has been reported to be as high as 30% in this subpopulation in the first year of life.
Although relatively rare, pulmonary vascular obstructive disease may occur in patients with long-standing substantial shunts and significant mitral regurgitation.
Children with trisomy 21 are at higher risk than the general population of developing pulmonary vascular obstructive disease at a younger age. Potential reasons for this include chronic upper airway disease, tonsillar and adenoid hypertrophy, and inadequate alveolarization of the terminal bronchioles, leading to a decreased surface area of the vascular bed.
Sex
The male-to-female ratio is 1:1.
Age
Patients with smaller defects and little or no mitral regurgitation may present at any age with a murmur and/or an abnormal ECG. Those with more severe mitral regurgitation typically present with CHF in the first 1-2 years of life.
Clinical
History
Children with smaller ostium primum atrial septal defects (ASDs) and little or no mitral regurgitation or left ventricle to right atrium shunting are usually asymptomatic. Those with significant pulmonary overcirculation and/or significant mitral regurgitation tend to present in infancy with congestive heart failure (CHF). Tachypnea and tachycardia are noted at rest and are exacerbated with crying or exertion. Feeding is accompanied by dyspnea, diaphoresis, and an increased work of breathing. The combination of feeding difficulties and increased metabolic demands results in failure to thrive, which may be severe and/or intractable.
Physical
Characteristic features of trisomy 21 may be detected. These include the following:
- Hypotonia and hyperflexibility
- Short, flat nose with a flat nasal bridge
- Oblique palpebral fissures
- Abundant neck skin
- Large and protuberant tongue
- Short, broad hands with a shorter fifth finger (clinodactyly)
- Simian crease
- Inner epicanthal fold extending onto the lower lid
- Brushfield spots (speckled iris)
Infants and children with partial atrioventricular (AV) septal defects and significant mitral regurgitation have poor development and are tachypneic and tachycardic at rest. A hyperinflated thorax, bulging precordium, and Harrison grooves are often present. Most children, however, have a milder degree of mitral regurgitation and, in general, appear normally developed and thriving on examination.
The cardiac examination in isolated ostium primum ASDs or partial AV canal defects with minimal mitral regurgitation is similar to that in other forms of ASDs. Patients typically have an increased right ventricular impulse secondary to volume overload. The first heart sound is normal. The second heart sound is fixed or at least widely split. A systolic ejection murmur is heard loudest at the upper left sternal border, with radiation to both lung fields. A click is not present. A tricuspid mid-diastolic rumble is present in children with larger shunts (pulmonary-to-systemic flow ratio > 2:1) and is appreciated at the lower left sternal border.
The murmur of mitral insufficiency is typically high-pitched, holosystolic, and loudest at the apex. The murmur usually radiates to the axilla but may radiate preferentially to the sternal edge secondary to streaming of the regurgitant flow across the atrial septum.
If pulmonary hypertension is present, significant changes are noted in the physical examination. The pulmonary component of the second heart sound becomes loud. The splitting of the second heart sound narrows and eventually may become single. The diastolic tricuspid rumble disappears. A holosystolic murmur of tricuspid regurgitation becomes noticeable as the right ventricle dilates. This murmur is usually loudest at the lower left sternal border and becomes higher pitched as the right ventricular pressure increases. A short midsystolic murmur may be present secondary to flow into a dilated pulmonary artery. A Graham-Steell pulmonary insufficiency murmur may be appreciated as an early diastolic decrescendo murmur at the mid left sternal border.
Causes
The most common cause of an ostium primum ASD is genetic, associated with trisomy 21. Well-described associations have been reported with Holt-Oram syndrome, Noonan syndrome, and Ellis-van Creveld syndrome, among others. In children with normal chromosomes, however, the cause remains unknown. Research into the molecular genetic basis for AV canal and AV septal defects is ongoing.
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References
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Further Reading
Keywords
atrial septal defect, ostium primum atrial septal defect, ASD, partial atrioventricular canal defect, partial atrioventricular septal defect, incomplete endocardial cushion defect, interatrial foramen primum, trisomy 21, Down syndrome, Down's syndrome, mitral regurgitation, heart murmur, congestive heart failure, congestive heart failure, CHF, pulmonary vascular obstructive disease, pulmonary hypertension, Holt-Oram syndrome, Noonan syndrome, and Ellis-van Creveld syndrome
